Low dispersion interdigital delay lines



May 14, 1963 R. c. HERGENROTHER 3,

LOW DISPERSION INTERDIGITAL DELAY LINES Filed Dec. 18, 1959 L5 1} 6) hamw l g g i Q 6) kg):

N VE N 7' 0R RUDOLF 0. HE R65 NRO THE I? B aw/M A TTORNEY United States 3 089,974 LGW DISPERSION INTERDIGITAL DELAY LINES Rudolf C. Hergenrother, West Newton, Mass, assignor to Raytheon Company, Lexington, Mass, a corporation of Delaware Filed Dec. 18, 1959, Ser. No. 860,426 5 Claims. (Cl. SIS-3.6)

' This invention relates to traveling wave tubes and more particularly to a low dispersion interdigital delay line for use in traveling wave tubes and the like wherein amplification of wave energy is effected by interaction between a traveling wave and a stream of electrons.

For successful operation of such tubes, it is necessary that the electron velocity and the phase velocity of the Wave to be amplified be approximately the same. The electron velocity depends upon the magnitude of the voltage used to accelerate the stream. Owing to the increase in mass with velocity, the additional voltage required to produce a given additional electron velocity will increase more and more rapidly as the velocity of light is approached. For this reason it is impracticable at present in tubes of the described type to use electron velocities greater than one quarter the velocity of light. Consequently, some means is required for propagating the wave at a correspondingly low velocity with respect to that of light. A typical prior art structure for this purpose comprises a helix which is supplied at one end with the input energy in such a manner as to carry a slowly traveling wave along its axis. The electron stream is directed along the longitudinal axis of the helix in the form of a beam, and amplified energy appears at the output end of the helix.

Another typical prior art structure for the same purpose comprises an interdigital delay line which is also supplied at one end with the input energy in such a manner as to carry a slowly traveling wave. The electron stream is directed along one or more sides of the delay line in the form of a beam, and amplified energy may in certain embodiments be taken at the output end of the line. However, inasmuch as the conventional interdigital delay line has a high dispersion it is most suitable for use in a backward wave oscillator since the wave travels in opposite directions in successive encounters with the electron beam thus resulting in a backward wave fundamental. For a forward wave amplifier it is desirable to have a non-dispersive delay line for a large bandwidth such as, for example, a helix. However, the helix ordinarily consists of many turns of relatively fine wire and must be supported to maintain its shape and pitch. On the other hand a tube utilizing an interdigital delay line has improved power handling capability but as pointed out hereinabove it is not generally suitable for use in a forward wave amplifier.

The present invention has for its object to provide an improved delay line or the like suitable for use in a forward wave amplifier.

In accordance with the invention, oppositely disposed and electrically connected portions of adjacent delay elements or fingers comprising a delay line are displaced or distorted toward each other along the longitudinal axis of the line whereby as between any two adjacent fingers oppositely disposed portions of each finger are in closer spatial relationship than the other two oppositely disposed portions. By reason of this configuration of the fingers successive wave paths are displaced in a direction perpendicular to the wave and perpendicular to the electron flow. As a result a single electron beam couples strongly to only every other Wave passage, thus in successive encounters with the electron beam only waves traveling in the same direction are coupled therewith. This results in a non or low dispersive fundamental mode similar, for example, to that of the helix and a delay line having the most desirable characteristics of the helix and interdigital type delay lines without their undesirable characteristics.

The above-mentioned and other features, advantages, and objects of the present invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a longitudinal cross section of a traveling wave tube illustrating the principles of the invention; and

FIG. 2 is a perspective view with parts broken away of a portion of the improved delay line to more clearly show the delay line structure.

Referring now to FIG. 1 which shows an 0 type forward wave amplifier, a delay line 11 constructed in accordance with the invention and more thoroughly described hereinafter is shown extending substantially throughout the major portion of the tube envelope 12. The tube envelope 12 is fabricated of a non-magnetic metallic material and is provided with insulative seals at convenient locations through which electrical connections are made to elements housed in the envelope. The delay line 11 is shown as an interdigital line. A volt-age source, here indicated by battery 13, establishes an electric field in the interaction spaces bounded by the delay line and the adjacent opposite Walls of the enclosure. A magnetic field is established in the enclosure 12 to prevent spreading of the electron beam or beams by any convenient means, such as, for example, a magnetic field produced by coil 14 surrounding the enclosure 12. Current is supplied to the coil by a suitable source, such as, for example, battery 15. At one end of the tube there are positioned dual electron guns symbolized by electron emitting cathodes 16 and accelerating electrodes 17 which are biased positively in conventional manner with respect to the cathodes 16 by the battery 18. A wall 19 having suitable apertures 21 is disposed between the accelerating electrodes 17 and the delay line 11. The cathodes 16 each enclose a heating element 22 which is connected to a suitable source of energy such as battery 23. Collector electrodes 24 are situated at the end of the tube opposite from the electron source. Input energy to the delay line 11 is provided by input coupling 25 connected to the delay line adjacent the gun end of the tube and the output of the tube is obtained from an output coupling 26 connected to the delay line adjacent the collector end of the tube.

Although a. dual electron gun has been shown it is to be understood that a dual gun of other construction or a single gun may be used with either an 0 type or M type tube construction.

With particular reference now to the delay line as shown in FIGS. 1 and 2 and as more clearly shown in FIG. 2, successive fingers of the delay line 11 are electrically connected at one end to opposite walls of the envelope 12 perpendicular to the electron flow and are of such length that their free ends are spaced from the wall adjacent thereto by a predetermined distance determinable in well-known and conventional manner. However, where the entire oppositely disposed surfaces transverse to the electron beam of the fingers of conventional interdigital lines lie in parallel planes, portions of adjacent fingers contemplated by the present invention are symmetrically equal and offset or staggered in opposite directions to form an abrupt step whereby as between any two adjacent fingers such as, for example, fingers Bil-32 two oppositely disposed portions 33-64 forming a part of one side of the line are spaced apart the required distance a but the remaining oppositely disposed end portions 35-36 on the other side of the line are symmetrically spaced apart a greater distance b which is equal to the distance a plus twice the finger thickness. This increased spacing b results in a spacing as between either one of the aforementioned adjacent fingers and the next adjacent finger of the required distance a on the side of the line opposite from that first referred to immediately hereinabove. As may now be apparent, the required or desired spacing of the fingers occur consecutively but on opposite sides of the line. Stating it another way, on each side of the delay line, all parts of which are symmetrically equal, the desireddistance a between consecutive but separate sets of fingers, for satisfactory coupling, is separated by a greater distance 11 such that little or no coupling between the wave and the electron beam occur at the point of increased separation. The distance a is determined in conventional manner for optimum coupling between the wave and the electron beam, the closeness of spacing controlling impedance and the configuration of the fingers controlling the relation between frequency and voltage. It will now be apparent that in accordance with the invention there are alternately provided weak and strong coupling between the wave and the electron beam, i.e., the appropriate electron beam is effectively coupled to every other wave passage and only waves traveling in the same direction successively encounter and are coupled to the electron beam as is shown in FIG. 1 where the circles with dot-s represent waves coming out of the drawing and the circles with a cross represent waves going into the drawing. It is this characteristic which results in a non or low dispersive fundamental mode and which permits utilization of an improved interdigital delay line in a forward wave amplifier and incorporates all of the advantages inherent in this and equivalent types of delay lines.

Although the invention has been shown in combination with an type tube it will be readily appreciated by those skilled in the art that it may be applied to an M type tube and that the invention is not limited to the specific applir cation or delay line shown and described. For example, either a single or dual electron beam may be used in M or 0 type tubes, the configuration of the fingers may vary from that shown and described so long as the fingers are symmetrically equal and the opposite portions are in electrical contact one with another, or the fingers may be formed with one or more axial passages through which an electron beam may be projected.

Since many changes could be made in the abovedescribed construction and many apparently widely different embodiments of the invention could be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a traveling wave electron discharge device the combination comprising: an electron source and electron beam forming means defining at least two paths of electron flow, one on each side at an elongate delay line comprised of geometrically periodic structure having delay elements disposed in coupling relationship with said path of electron flow, each of said delay elements being comprised of two portions transverse to said path of electron flow and displaced one from another to form an abrupt step in the direction of the longitudinal axis of said line, one pair of oppositely disposed portions of adjacent delay elements being displaced toward each other with respect to the other portions of said adjacent delay elements.

2. In a traveling wave electron discharge device the combination comprising: an electron source and electron beam forming means defining at least two paths of elec tron flow, one on each side of an elongate delay line comprised of geometrically periodic structure having delay elements disposed in coupling relationship with said path of electron flow, each of said delay elements being comprised of two portions transverse to said path of electron flow and displaced one from another in the direction of the longitudinal axis of said line to form an abrupt step, one pair of the oppositely disposed portions of adjacent delay elements being displaced toward each other with respect to the other pair of oppositely disposed portions of said adjacent delay elements, each of said other pairs of portions being respectively displaced toward the delay elements next adjacent thereto.

3. In a traveling wave electron discharge device the combination comprising: an electron source and electron beam forming means defining at least two paths of electron flow, one on each side of an elongate delay line comprised of geometrically periodic structure having delay elements disposed in coupling relationship with said path of electron flow, each of said delay elements being comprised of two electrically connected portions transverse to said path of electron flow and offset one from another in the direction of the longitudinal axis of said line, a first pair of oppositely disposed portions of adjacent delay elements being offset toward each other with respect to the second pair of oppositely disposed portions of said adjacent delay elements to form abrupt teps with said econd pair, said first pair of oppositely disposed portions eing spaced apart a predetermined distance for maximum coupling, each of said second pair of oppositely disposed portions being respectively offset toward the delay elements next adjacent thereto for minimum coupling between said second pair and maximum coupling between each of said second pair and the delay element next adjacent thereto.

4. A traveling wave electron discharge device comprising: an evacuated housing; at least one electron source and electron beam forming means located in said housing defining at least two paths of electron flow, one path on each side of an elongate interdigital delay line located in said housing comprised of geometrically periodic structure having delay elements to enable interaction between a radio frequency wave traveling therealong and said electron flow, each of said delay elements being comprised of two electrically connected portions transverse to said path of electron flow and offset one from another in the direction of the longitudinal axis of said line to form an abrupt step a first pair of oppositely disposed portions of adjacent delay elements being offset toward each other with respect to the second pair of oppositely disposed portions of said adjacent delay elements, said first pair of oppositely disposed portions being spaced apart a predetermined distance for maximum coupling, each of said second pair of oppositely disposed portions being respectively offset toward the delay elements next adjacent thereto for minimum coupling between said second pair and maximum coupling etween each of said second pair and the delay element next adjacent thereto; means for applying wave energy to one end of said delay line to induce propagation of said wave along said delay line in the direction of electron travel; and means for extracting said wave from the other end of said delay line.

5. An elongate delay line comprising: a geometrically periodic structure having delay elements disposed in coupling relationship with at least two paths of electron flow, one of said paths on each side of said structure, said delay elements being comprised of a first group and a second group, each of said second group being interposed between each of said first group, each of said delay elements spatial relationship defining an energy storage gap therebeing comprised of two symmetrical and electrically conbetween whereby successive wave paths between adjacent nected portions offset one from another in the direction delay elements are displaced in a direction transverse to of the longitudinal axis of said line to form an abrupt the Wave and coplanar with the wave.

step, one portion of each of said delay elements of said 5 first group being in closer spatial relationship than its References Cited In the file of this Patel?!t other portion to the next succeeding adjacent delay ele UNI D STATES PATENTS ment of said second grou and one portion of said next succeeding delay element lacing in closer spatial relation- 21888595 Warnecke et a1 May ship than its other portion to the aforementioned delay 10 2889486 Guenard et a1 June 1939 element in said first group, the portions in said closer 219161656 Moms et a1 1959 

1. IN A TRAVELING WAVE ELECTRON DISCHARGE DEVICE THE COMBINATION COMPRISING: AN ELECTRON SOURCE AND ELECTRON BEAM FORMING MEANS DEFINING AT LEAST TWO PATHS OF ELECTRON FLOW, ONE ON EACH SIDE AT AN ELONGATE DELAY LINE COMPRISED OF GEOMETRICALLY PERIODIC STRUCTURE HAVING DELAY ELEMENTS DISPOSED IN COUPLING RELATIONSHIP WITH SAID PATH OF ELECTRON FLOW, EACH OF SAID DELAY ELEMENTS BEING COMPRISED OF TWO PORTIONS TRANSVERSE TO SAID PATH OF ELECTRON FLOW AND DISPLACED ONE FROM ANOTHER TO FORM AN ABRUPT STEP IN THE DIRECTION OF THE LONGITUDINAL AXIS OF SAID LINE, ONE PAIR OF OPPOSITELY DISPOSED PORTIONS OF ADJACENT DELAY ELEMENTS BEING DISPLACED TOWARD EACH OTHER WITH RESPECT TO THE OTHER PORTION OF SAID ADJACENT DELAY ELEMENTS. 